These days, there are several types of optical disks in which information is recorded, erased and reproduced by a laser beam, which include the optical magnet and phase-change types of optical disk. In a phase-change type of optical disk, light is irradiated onto the surface of the disk to change the phase condition to record information, erase the recorded information and reproduce the recorded information.
Usually a phase-change type of optical disk includes a substrate, a lower protection layer provided on the substrate, a recording layer provided on the lower protection layer, an upper protection layer provided on the recording layer and a reflection layer provided on the upper protection layer. When the recording layer is heated so as to be melted and suddenly cooled down so as to become non-crystal condition, information is recorded thereon. On the other hand, when the recording layer is maintained at a predetermined temperature so as to have a crystalline condition, the recorded information is erased. The recorded information can be reproduced by using the difference in reflection rate between crystal region (non-recorded) and non-crystal region (recorded).
For high density recording, a mark-edge recording by which information is recorded on the both ends of a recording mark is useful. In a phase-change type of optical disk, crystal and non-crystal regions are different in absorbing rate from each other. When overwriting information, the length of a record mark changes depending on the condition of the recording layer, i.e., whether crystal or non-crystal. To prevent this kind of problem, an optical disk having a reflection layer which is of Si or Au having a thickness of less than 20 nm has been proposed to increase the absorbing rate of the crystal region above that of the non-crystal region. DC erasing can be used for the same purpose, however, overwriting cannot be carried out.
The phase-change type of optical disk which has been just fabricated has a recording layer of non-crystalline condition, so that the disk is required to be initialized before use. For initializing such a disk, the recording layer is crystallized by irradiating laser beam or flash lamp thereto. There have been proposed some conventional methods for initializing a phase-change type of optical disk, that are described in Japanese Publication Kokai H3-278338 and H4-209317 to increase erasing rate and C/N or to shorten initializing time. In publication H3-278338, a laser beam having a larger diameter is used to maintain time for crystallization. In publication H4-209317, the recording layer has inconsistent composition. According to the conventional technology in a phase-change type of optical disk, recording characteristics change between the beginning, which is just after initialization, and after some re-writing. Especially, in mark edge recording, recording error is increased.